1. Field of the Invention
The present invention relates to thermistor devices and, more particularly, to a positive-characteristic thermistor device used in a demagnetizing circuit incorporated in a TV receiver and also to a negative-characteristic thermistor device used in a temperature-compensating circuit or the like.
2. Description of the Prior Art
A known thermistor device having a positive or negative temperature coefficient is shown in FIGS. 7 and 8. The body of the thermistor is indicated by numeral 30. Electrodes 31 and 32 made from a conductive material consisting mainly of silver (Ag) are formed on the front and back surfaces, respectively, of the thermistor body 30. The electrodes 31 and 32 are in ohmic contact with the thermistor body 30.
In the thermistor device of this construction, if a potential difference is developed between the electrodes 31 and 32, some Ag atoms forming the material of the electrodes 31 and 32 migrate across the surface of the thermistor body 30, thus deteriorating the insulating performance. In the worst case, the electrodes 31 and 32 are shorted together. Referring to FIG. 8, A and D refer to the outer ends of the electrodes 31 and 32, respectively, and B and C refer to the left and right edges, respectively, of the outer end surface of the thermistor body 30. Because of the resistive component of the thermistor body 30, potential differences are produced between A and B, between B and C, and between C and D on the surface of the thermistor body 30. These potential differences cause migration of the Ag atoms forming the electrodes 31 and 32.
Another thermistor device equipped with means for reducing or slowing this problem has been proposed, and is shown in FIGS. 9 and 10. This thermistor device is similar to the known thermistor device already described in conjunction with FIGS. 7 and 8 except that the surface of the thermistor body 30, excluding the portions covered by the electrodes 31 and 32, is coated with an insulating film 33 made of a resin, glass, or the like. As shown in FIGS. 9 and 10, the Ag migration entails the movement of metal caused by a potential difference between A and B, between B and C, and between C and D. In addition, if there is a potential difference, the migration velocity is accelerated when the thermistor device is operated in a moist atmosphere, and the electrolytic ion such as chloric ions, sulfurate ions, or the like are absorbed onto the thermistor surface on operating. Coating the thermistor body with resin or glass will prevent water and the electrolytic ions from being absorbed onto the thermistor surface, thus maintaining the migration at a low velocity.
However, it is costly to fabricate this thermistor device shown in FIGS. 9 and 10, because it is cumbersome to coat the outer surface of the thermistor body 30 with the insulating film 33 made of a resin or glass.